Artificial Vision.
Researchers report progress on retinal implant.

SAN FRANCISCO — MIT and medical researchers are working on a chronic retinal implant in hopes of restoring vision to those suffering from age-related macular degeneration and retinitis pigmentosa.

"The goal is to stimulate the remaining healthy layers of retinal neurons using brief biphasic current pulses," reported researcher Luke Theogarajan, at the International Solid-State Circuits Conference here.

Indeed, progress in the miniaturization of retinal and cochlear prosthesis was reported at the conference. For the more than 14 million people worldwide suffering from these vision problems, or from hard of hearing, the news is encouraging. Still, application in humans is rare.

Blindness results from loss of photoreceptors, but other retinal neurons maintain an active connection to the brain. Researchers from the Massachusetts Institute of Technology and from Massachusetts Eye and Ear Infirmary at Harvard University noted that the design requirements for the implant include an external power source; wireless communication of external commands to the implant; and the ability to provide wireless tuning of pulse amplitude, duration and inter-pulse timing.

The source and wireless requirements were met by an inductively coupled power and data link. A flexible stimulator chip architecture was developed that allows for seamless scaling of the number of electrodes that can be physically implanted in a minimally invasive approach.

Only the electrode array is placed in the eye beneath the retina, while the secondary coils and stimulator chip are surgically attached to the eyeball. "Surgical trauma to the eye is greatly minimized by placing the bulky electronics in the more compliant eye socket, rather than the delicate retina," said Theogarajan.

The implant was made from a paralyene-encapsulated flexible polyimide substrate onto which the chip and electrode array are stud bump-bonded. The stimulator chip architecture provides frequency-independent operation and compensates for non-idealities due to process, temperature and voltage. Power from an inductive link is rectified and filtered using off-chip diodes and capacitors housed on the flexible substrate, with the 2.5-V supply voltage regulated by an off-chip zener diode.

Nominal carrier frequencies for power and data are 125 kHz and 13.56 MHz, respectively. The chip can drive 15 electrodes, contains 30,000 transistors in a 0.5-micron technology, occupies 2.3 by2.3 square millimeters, and consumes 1.3 mW at a data rate of 100 kbits/second (excluding the current sources).

Meanwhile, researchers from two German companies, design house sci-worx and medical electronics firm IIP Technologies, are working on an epiretinal prosthesis, which restores basic vision through electrical nerve stimulation within the eyeballs of blind people with retinal degeneration. Electrical stimulation generates a nerve reaction upon the transfer of charge into the tissue via electrodes. Common architectures use global generation of stimulation currents and distribution over current switches, a process that allows only one or a small number of electrodes to be activated at a time.

Eyes and ears

The German design presents the concept of an array of fully digitally interfaced and programmable stimulation pad cells for a retinal implant in 0.35-micron HVCMOS, which has a maximum voltage swing of 15 V, includes full-custom ESD protection and an innovative active charge balancer.

"Currently, a complete retinal stimulator chip is being fabricated, which includes all global functions and 116 stimulation pad cells," said the IIP Technologies researchers. Clinical trials have started.

As for hearing-impaired cochlear prostheses, they have evolved significantly during the past 20 years. More than 90,000 such devices have been implanted to date, restoring functional hearing to many profoundly deaf and severely hearing-impaired patients. A cochlear implant bypasses the failed hair cells of the inner ear to electrically stimulate the auditory nerve using 16 to 22 wire electrodes. Although such implants have been remarkably effective, speech perception among patients varies widely, and there are difficulties in understanding tonal languages and appreciating music.

Researchers at the University of Michigan see a potential solution to these problems in the development of electrode arrays that increase the numbers of stimulating sites so that the arrays can more easily adapt to differing patterns of nerve survival and use multi-polar current shaping to increase pitch perception.

Increasing the number of wire electrodes is precluded by the size of the cochlea, which tapers from a diameter of about 1 mm to about 200 m over its length. Integrated array position sensors to help optimize array placement and minimize insertion damage are also needed.

Researcher Pamela Bhatti reported on a thin-film cochlear electrode array that achieves high site density and incorporates on-board circuitry for stimulus generation and position sensing. "The array is designed for use in guinea pig studies but offers the same features and site densities needed for a 128-site, 16-channel human array," said Bhatti.

The 8-mm-long substrate tapers from a width of 500 to 200 microns and supports thirty-two 180-micron-diameter iridium-oxide stimulating sites on 250-micron centers. A 14-micron-thick, boron-diffused silicon area forms the base of the array, which remains outside the cochlea to support circuitry for current generation, site selection and position sensing. An eight-lead polymeric cable connects the array to an hermetically sealed electronics package containing a microcontroller along with a wireless interface for power and bidirectional data transfer.